Cysteine proteinases containing a highly reactive cysteine residue with a free thiol group at the active site have been known as playing important role in certain conditions distinguished by aberrant protein turnover such as: muscular dystrophy (Am. J. Pathol. 1986, 122, 193-198, Am. J. Pathol. 1987, 127, 461-466), bone resorption (Biochem. J. 1991, 279, 167-274), myocardial infarction (J. Am. Coll. Cardiol. 1983, 2, 681-688), cancer metastasis (Cancer Metastasis Rev. 1990, 9, 333-352) and pulmonary emphysema (Am. Rev. Respir. Dis. 1975,111, 579-586). A variety of cysteine proteinases have been shown to be present in mammalian tissue. The most notable of these proteinases are the lysosomal cathepsins (cathepsin B, H, S, and L) and the cytoplasmic Ca.sup.2+ dependent enzymes, the calpains. These enzymes are, therefore, excellent targets for the development of specific inhibitors as possible therapeutic agents for the conditions such as those noted above.
Cysteine proteinases are inhibited by several types of peptide derived inhibitors such as peptidyl aldehyde (Eur. J. Biochem. 1982, 129, 33-41), chloromethyl ketone (acta. Biol. Med. Ger. 1981, 40, 1503-1511), diazomethyl ketone (Biochemistry 1977, 16, 5857-5861), monofluoromethyl ketone (Biochemical Pharmacology 1992 44, 1201-1207), acyloxy methyl ketone (J. Med. Chem. 1994, 37, 1833-1840), O-acyl hydroxamates (Biochem. Biophy. Research Communications 1988, 155, 1201-1206), methyl sulphcnium salts (J.
Biol Chem. 1988, 263, 2768-2772) and epcxy succinyl derivatives (Agric. Biol. Chem. 1978, 42, 523-527) which do not significantly inhibit other classes of proteinases.
These inhibtors, in general, have peptidyl affinity groups and reactive groups towards the thiol of the cysteine residue of cysteine proteinase. Some of the inhibitors are clinically useful. However, their effectiveness in vivo is not as much as expected on the basis of in vitro inhibitory actvity, perhaps due to lower selectivity towards other proteinases and poor pharmacokinetics. Therefore, there exits a continuing need to develop new cysteine proteinase inhibitors with high selectivity and lower toxicity. ##STR2##